Robert Gonzalez wrote:
I way I interpreted the photo.net explanation it sounds like the magnification factor has an effect on the DOF. Since it takes more magnification for a smaller sensor to fill the 8x10, the DOF will be different. In what you are saying, it sounds like M, the magnification factor is effectively the same since you are getting the same final crop in terms of subject sizes? Here is a summary of what that web site has (I have substitued the *istD for his original example):
1. For an equivalent field of view, the *istD has at least 1.5x MORE depth of field that a 35mm film camera would have - when the focus distance is significantly less then the hyperfocal distance (but the 35mm format need a lens with 1.5x the focal length to give the same view).
True. If you used a 24mm on the istD and a 35mm on the film camera the images would have about the same magnification, but the aperture on the 35mm lens would be 1.5 times as large so you would have less DOF on the film camera.
2. Using the same lens on a *istD and a 35mm film body, the *istD image has 1.5x LESS depth of field than the 35mm image would have (but they would be different images of course since the field of view would be different)
Here you have 3 ways of working if you want the same image.
1. The simplest would be to move closer with the film camera so the subject is the same size in the final print as it is with the digital camera. In that case the magnification is the same and the DOF is the same (remember the focal length and f-stop remain same also).
2. You could take the photos from the same distance and crop the 35mm image to match the digital image or you could make a larger print from the 35mm so the subject is the same size as in the digital but it shows more of the surroundings in both cases magnification and aperture remain the same and the DOF does not change.
3. You could make prints in which the subjects are of different sizes. In which case the magnification is less with the 35mm and the DOF will be less with the digital (presumably this is the case intended in the above statement.
3. If you use the same lens on a *istD and a 35mm film body and crop the 35mm image to give the same view as the digital image, the depth of field is IDENTICAL
True. This is my case 2 above.
4. If you use the same lens on an *istD and a 35mm film body, then shoot from different distances so that the view is the same, the *istD image will have 1.5x MORE DOF then the film image.
False. In this case the overall magnification remains the same, the aperture remains the same. Therefore the DOF remains the same. This is my case 1 one above.
Of course if you going to look at a sensor sized image then his statement is correct. But he is forgetting that you have to enlarge the digital image more to get the same size print. The increased magnification cancels out the longer focus distance.
5. Close to the hyperfocal distance, the *istD has a much more than
1.5x the DOF of a 35mm film camera. The hyperfocal distance of the *istD is 1.5x less than that of a 35mm film camera.
False again, if he is still using the same lenses. If using equivalent focal lengths (say the 24 & 35 I mentioned) then he is correct, but not for his reasons. It is once again because the diameter of the aperture on the 35mm is 1.5x the aperture on the 24mm lens.
I'm not sure, but I think what you are saying is consistent with this, correct?
Mostly.
It took me almost 20 years to realize it is the diameter of the aperture and not the f-stop that matters. I do not think this guy has figured that out. The diameter of the aperture controls a lot of things in optical calculations, but us photographers are so immured in f-stops we tend to think they are the same thing. They are not, f-stop is focal length divided by aperture and it is an approximate light transmission figure.
rg
graywolf wrote:
DOF does not have much to do with the size of the Circle of Confusion (COC) on the film or sensor unless you only look at contact prints. Normally DOF is based on an 8x10 print viewed at 10 inches.
When you reduce the formulas to their basics the only things that matter is COC in the final image, the size of the subject in the final image (magnification M), and the diameter of the aperture A (not f-stop). If you use a uncropped 8x10 @ 10 inches as your reference COC becomes a constant.
What that means is in that the same size subject in an 8x10 print, a given f-stop (f4.5 in the mentioned case) with a given lens (24mm NOTE: you have to specify the focal length if you use f-stop, because what is involved in DOF is the diameter of the aperture and you need the focal length of the lens to convert f-stop to aperture) has exactly the same DOF with the small sensor as it does with 35mm film. That applies whether you move closer to fill the larger film frame or crop down to match the smaller sensor as both methods give the same overall magnification.
I repeat, in the final image the DOF is exactly the same with both formats.
Dario Bonazza 2 wrote:
Good point. The 24mm should become a 36mm, while depth-of field must be
conidered one stop less, hence pictures taken with the 24mm f/4.5 are
like
those taken at 36mm f/3.5, while 24mm f/11 is like 36mm f/8. However, I was expecting some more sharpness there (not more unsharp
mask!).
Can you explain your logic here? In my experience the DOF is based on the lens focal length, not the 35mm equivelent focal length.
The 24mm on the *ist D gives you the field of view of a 36mm lens but the depth of field of a 24mm lens (because that is what it is). A 36mm lens at f3.5 would have much less depth of field.
Glad you noticed that. I'll try to explain this concept (all but mine).
The depth of field is based on the concept of confusion circle:
1) Your eyes see as pinpoint each spot size below their resolution.
2) Your eyes can appreciate dimensions of each spot size above their
resolution.
Images look unsharp (out of focus) when the size of dots forming them are
above eye resolution and look sharp (in focus) when dots forming them are
below that limit.
The confusion circle is a parameter in optical design, and depth of field as
indicated in DOF scales is related to it.
Since *ist D CCD sensor diagonal is 1.5 times smaller than that of 35mm
film, using a 24mm designed for film on such a sensor not only gives you an
angle of view equivalent to that of a 36mm lens, but in order to do that it
only uses (enlarging it) a central portion of its possible image field.
So, to get a print (or file as seen on PC monitor) the same size of that
taken with a "true" 35mm lens on a 24x36mm sensor, you have to enlarge the
image of such a 1.5 factor.
Think of doing that with film: should you want to get a 35mm perspective
print out from a 24mm slide or negative, you have to enlarge it 1.5 times
more and then crop the print to the same size of that made with a true 35mm
lens. The only difference is that the *ist D crops during shooting.
You'll enlarge the image taken with the 24mm more than that shot with the
true 35mm on a larger sensor (35mm film format), hence you'll push image
resolution of the 24mm to a 1.5x higher extent. In other words, some dots
which would stay below eye resolution when sooting on 35mm format will jump
out, and a lesser part of the image will look in focus.
The 1.5x ratio between the two sizes (35mm film and CCD) roughly correspond
to 1 stop wider.
I dindn't invent such theory. It's explained (not so well) in *ist D
instruction manual (see bottom of page 137).
Not sure if I explained well in my poor English.
Dario Bonazza
-- graywolf http://graywolfphoto.com
"You might as well accept people as they are, you are not going to be able to change them anyway."
